Add oauth2Login MockMvc Test Support

[jzheaux]

Related to #7618

When testing applications that use OAuth 2.0 Login, it would be nice to have MockMvc support like:

this.mvc(get("/")
    .with(oauth2Login()));

or

this.mvc(get("/")
    .with(oauth2Login()
        .attribute("name", "value")
        .authorities(new SimpleGrantedAuthority("SCOPE_read")));

as well as other builder methods relative to generating an authentication statement around an OAuth2User principal.

Note that the attribute method is necessary since the OAuth 2.0 spec doesn't specify any particular claims. It, then, correlates to the idToken and userInfo methods in the oidcLogin() MockMvc support.